Urotensin-II (U-II) is a vasoactive, somatosatin-like cyclic peptide (Coulouarn et al., 1999, FEBS Lett 457(1): 28-32). U-II was originally isolated from the teleost urophysis, and was shown to be involved in the cardiovascular regulation, osmoregulation, and regulation of lipid metabolism in fish (Ohsaka et al., 1986, J. Neurosci 6:2730-2735; and Conlon et al., 1996, J. Exp. Zool. 275:226-238). The genes encoding orthologs of U-II precursor proteins have since been cloned from various species, for example, rat (Marchese et al., 1995, Genomics 29: 335-344), human (Coulouarn et al., 1998, Proc. Natl. Acad. Sci. U SA 95: 15803-15808; and Ames et al., 1999, Nature 401(6750): 282-6), and mouse (Coulouarn et al., 1999, supra). Human U-II is found within both vascular and cardiac tissue (including coronary atheroma). In addition, U-II immunoreactivity is also found within central nervous system and endocrine tissues (Ames et al., supra).
G-protein-coupled receptor 14 (GPR14), also known as sensory epithelium neuropeptide-like receptor (SENR), was recently identified as to function as an U-II receptor (Ames et al., supra). GPR14 was cloned as an orphan receptor with similarity to members of the somatostatin/opioid family. Human U-II binds to recombinant human GPR14 with high affinity and the binding is functionally coupled to calcium mobilization. The receptor of U-II (UT receptor) has also been identified and characterized from other animals, for example, mouse and monkey (Elshourbagy et al., 2002, Br. J. Pharmacol. 36: 9-22). The UT receptor is expressed abundantly in the spinal cord, and also in heart, lungs, blood vessels, kidney, and brain (Russell, 2004, Pharmcology & Therapeutics 103: 223-243).
Studies have demonstrated that U-II is both an endothelium independent vasoconstrictor (Ames et al., supra; Maguire et al., 2000, Br. J. Pharmacol. 131(3): 441-6] and an endothelium dependent vasodilator (Bottrill, 2000; Br. J. Pharmacol. 130(8): 1865-70; Zhang et al., 2003, Am. J. Physiol. Renal. Physiol., 285, F792-8). The vasomotor profile of U-II exhibits significant species differences, as well as regional and functional differences between vessels (Douglas et al., 2000, Br. J. Pharmacol. 131(7): 1262-74). At higher concentrations, U-II induced a sustained vasodilation that was significantly inhibited by a cyclooxygenase inhibitor (Katano et al., 2000, Eur. J. Pharmacol. 402(1-2): R5-7). When directly administrated into the renal artery, U-II increased renal blood flow as well as diuresis and naturesis in a dose-dependent manner, suggesting that U-II may produce renal vasodilation. These effects were abolished by L-NAME (Zhang et al., supra), the nitric oxide inhibitor. L-NAME increases the vasocontractile response of U-II (Maguire et al., supra). In isolated perfused rat heart, U-II elicited a concentration-dependent increase in coronary resistance (Gray et al., 2001, Life Sciences 69(2): 175-180). In the presence of L-NAME and the cyclooxygenase inhibitor, indomethacin, U-II significantly increased the coronary perfusion pressure three-fold, suggesting that U-II mediates the release of the vasodilators, nitric oxide and prostacyclin (Gray et al., supra).
Emerging roles of U-II in cardiovascular diseases have been implicated (Russell k supra). Recent evidence suggests that the UT receptor system is up-regulated in multi-organ disease states, such as congestive heart failure (CHF), pulmonary hypertension, and chronic renal failure. A number of non-peptide UT receptor antagonists have been developed with the aim of dampening harmful effects of over-activated UT receptors (see, i.e., Douglas et al, 2004, Trends. Pharmacol. Sci. 25: 76-85).
To facilitate the development of new compounds that regulate the biological activity of the UT receptor, there is a need to develop methods that allow simple measurement of the ability of a candidate compound to increase or decrease the biological activity of UT receptor.